Recently, in the field of composite materials for dental use, not only a strength and hardness equivalent to those of natural teeth, a surface smoothness, and resistance against abrasion as caused by teething, but also such factors as compatibility in color tone with natural teeth and conformity in a refractive index for providing transparency equivalent to that of natural teeth are required. Furthermore, ordinary materials for dental use are required to have an X-ray radiopacity enabling differentiation of a portion treated or repaired with the materials from a tooth tissue of natural teeth, and in addition the materials are required to have easiness in processing for dentists or dental technicians.
As an example of the composite materials for dental use, for instance, Patent document 1 discloses a composite material for dental use comprising a polymeric resin and non-vitric microparticles (i.e., a mixture of polycrystalline ceramic metal oxide and amorphous silicon oxide) with the X-ray radiopacity. However, because the non-vitric microparticles contain a polycrystalline ceramic metal oxide, it is difficult to obtain the transparency equivalent to that of natural teeth. On the other hand, when the microparticles containing the metal oxide prepared by heating at a temperature of not causing crystallization of the metal oxide are employed for acquiring the transparency, the strength of the microparticles is insufficient, and if the composite material contains such microparticles, the strength and the hardness at the treated or repaired portion of a tooth (including an artificial tooth) are lowered, and sometimes a degree of the resistance against abrasion as caused by teething becomes insufficient.
Patent document 2 discloses a dental filler for producing a composite material, which comprises silicon dioxide and other metal oxide each forming an independent amorphous layers prepared by aggregating particles of silicon dioxide and at least the other metal oxide (e.g., zirconium oxide or the like) and heating them at a temperature of lower than crystallization temperatures of the oxides. However, because the dental filler is prepared by aggregating the particles of silicon dioxide and other metal oxide, the pore volumes of the aggregated particles and also the strength thereof are not adjustable to a desired value. Therefore, it is difficult to improve the transparency at the treated or repaired portion of a tooth (including an artificial tooth). Furthermore, as the adhesion of the aggregated particles to the polymeric resin is insufficient, the strength and the hardness at the treated or repaired portion of a tooth (including an artificial tooth) are lowered, and sometimes a degree of the resistance against abrasion as caused by teething becomes insufficient, like in the case described above.
Furthermore, 3M Innovative Properties Company in the United State made a series of patent applications for materials for dental use (Refer to Patent documents 3 to 5).
Patent document 3 discloses a dental material containing a) a hardenable resin, and b) a filler containing (i) nano size particle cluster containing non-heavy metal oxide particles (such as silica particles) and heavy metal oxide particles (such as zirconium oxide particles), which is not fully densified, and (ii) not-aggregating non-heavy metal oxide nano-sized particles or not-aggregating heavy metal oxide nano-sized particles. The nano-sized particle cluster is substantially amorphous, and is used to improve strength of dental materials, and in addition the nano-sized particles are used for providing beauty, glazing property, and improving the abrasion resistance. When this material is used, however, because the filler has not been fully densified, the compression strength of dental materials using such filler is generally regarded as insufficient, although the tensile strength thereof is improved.
Patent document 4 discloses a dental material containing non-heavy metal oxide particles having the average particle diameter of less than about 300 nm, a heavy metal oxide or heavy metal oxide particles having the average particle diameter of less than about 100 nm, and a hardenable resin. The document includes the descriptions that spherical silica particles or an aggregated material thereof is used as the non-heavy metal oxide particles and that crystalline zirconium oxide particles having a high refractive index and a high X-ray scattering capability or an aggregated material thereof is used as the heavy metal oxide particles. When the material is used, however, because a refractive index of the heavy-metal oxide particles is higher than that of the hardenable resin, a degree of the light scattering inside the material becomes larger and the material may be whitened. As a result, the effect of generating beauty may be rather deteriorated, although the glazing property is improved.
Patent document 5 discloses a dental filler containing substantially amorphous cluster containing non-heavy-metal oxide particles (such as silica particles) having the average particle diameter of less than about 100 nm and heavy-metal oxide particles (such as zirconium oxide particles) having the average particle diameter of less than about 100 nm. In this case, the substantially amorphous cluster is essentially free of crystalline structure. However, because the filler comprises the substantially amorphous cluster, the mechanical strength (especially, the compression strength) of the portion treated or repaired with use of such filler may not be sufficient, although the required mechanical strength varies dependent upon its use or application.
On the other hand, Non-patent document 1 discloses a method of synthesizing a wadeite compound (K2ZrSi3O9.H2O) by mixing SiO2 (dissolved in KOH solution) with a Zr(OC3H7) solution, putting the mixture solution in a stainless steel Teflon™ vessel and then carrying out a reaction between the above components at the temperature of 180° C. for 5 days. In this document, however, only physical properties of the synthesized wadeite compound and the like are described in detail, and there is not even a suggestion that the wadeite compound is applicable or useful as a material for a dental use.
The present inventors devoted themselves to solve the problems in the conventional filler for dental use as described above, and found that zirconium silicate compounds have excellent properties as a dental filler, and completed the present invention.
Patent document 1: JP S60-233007A
Patent document 2: JP H07-196428A
Patent document 3: JP 2003-512404A (WO01/030304)
Patent document 4: JP 2003-512405A (WO01/030305)
Patent document 5: JP 2003-512406A (WO01/030306)
Non-patent document 1: Inorganic Chemistry Vol. 36, No. 14 (1997), Pages 3071-3079